The amount of organic matter in soils—decomposed leaves, decayed plant roots, and communities of soil microorganisms—determines how well a particular soil will sustain plant growth. Soil Organic Matter (SOM) decomposes with time to form carbon dioxide, methane, and nitrous oxides (greenhouse gases), but the decomposition speed depends on environmental conditions, such as soil moisture, temperature, and composition, as well as the composition of the microbial community. Scientists puzzled over the role that soil minerals play in keeping organic matter stable, particularly in arid soils, which make up nearly one-third of all soils. Calcite, the most common mineral form of calcium carbonate in these arid soils, appears to hold the key in the stabilization of SOM, though interactions with other soil constituents are far more complex than originally thought.
SOM represents the world’s largest reservoir of carbon, containing more than three times as much as found in the atmosphere or terrestrial vegetation. Holding that carbon stable not only keeps it out of the atmosphere, where it forms a harmful gas that contributes to climate change, it also supports healthy plant growth. Scientists identified a link between the amount of calcium in arid soil and the amount of SOM. Understanding the processes involved—whether in dissolving calcite, releasing calcium, or binding soil with organic compounds—provides insights into approaches that could be used to manage organic matter in soils.
A multi-disciplinary team of scientists studied the complex nature of SOM and its association with the soil minerals that are abundant in arid environments. They studied the interaction of calcite, the most common mineral form of calcium carbonate, with four compounds that represent major classes of soil organic matter—fatty acids, amino acids, carbohydrates, and lignin—at pH values relevant for arid environments. Starting with molecular-level modeling of possible interactions, the team conducted experiments that allowed them to consider interactions of organic matter and calcite at the molecular level. To view the detailed mechanisms by which organic matter interacts with minerals, they used advanced instruments that provided atomic-scale information, including a nuclear magnetic resonance spectrometer and a scanning transmission electron microscope, which are available through EMSL, the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy (DOE) Office of Science User Facility. Their observations proved that calcite’s interactions with different SOM components and mixtures depend on the organic compounds involved. These interactions are a function of two processes: 1) the dissolution of calcite, which releases calcium ions; and 2) the complexation of those released ions with organic compounds. The findings will help lay the foundation for how to stabilize organic matter in soils.
Odeta Qafoku, Environmental Molecular Sciences Laboratory, Odeta.Qafoku@pnnl.gov
Ravi Kukkadapu, Environmental Molecular Sciences Laboratory, firstname.lastname@example.org
Amity Andersen, Environmental Molecular Sciences Laboratory, email@example.com
A portion of this research was performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility.
O. Qafoku, et al., “Selective interactions of soil organic matter compounds with calcite and the role of aqueous Ca.” ACS Earth and Space Chemistry 6, 1674 (2022). [DOI: 10.1021/acsearthspacechem.2c00016]